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Roger Boisjoly-The Challenger Disaster



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Year 1987
Language English

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What Went Wrong

The Challenger on the morning of its final launch

Why was Roger Boisjoly so concerned about O-Rings? These seemingly insignificant pieces of rubber played a critical role in the joints between segments of a solid rocket boster (SRB ).

The two SRBs attached to a space shuttle orbiter provided eighty percent of the thrust necessary to propel the shuttle into space. About two minutes after a normal launch, the SRBs would detach and parachute back to the ground to be reused in subsequent missions. Several cylindrical segments make up the 149.1-foot- (45.4-meter-) tall SRB. Each joint between these segments contains two O-rings, positioned concentric with the SRB. The O-rings must be in perfect condition to prevent hot gasses from leaking through the joints of the SRB.

The Challenger: What Went Wrong

Within a second of the launch of Challenger on January 28, 1986, the first signs of failure of a joint in the right SRB were visible. Puffs of black smoke, whose color suggested that 5800-degree gases were eroding the O-rings, spewed out of that joint three to four times each second. At the end of the first minute, a small but steady flame was evident.

The small flame breaches the external tank.

Atmospheric and aerodynamic conditions directed the flame plume onto the surface of the External Tank, used to supply liquid hydrogen and liquid oxygen fuel to the shuttle's engines during the launch. The flame eventually breached the tank, and a massive amount of hydrogen and oxygen burst into flame. At 73 seconds, a nearly explosive burn of the hydrogen and oxygen quickly resulted and claimed the Challenger with its crew.

Solid Rocket Booster laying on it's side

Above: A Solid Rocket Booster and its segments. The aft field joint (arrow) failed in the Challenger's right SRB.

Diagram of a solid rocket booster's motor joint (where the O-Ring is)

A Solid Rocket Motor Joint. Its parts are colorized in this diagram for clarity. In pink is the tang, which joins the clevis, colored orange. 177 huge steel pins (yellow) hold the joint in place. The O-rings shield the joint from 5800-degree gases inside the booster.

On the left scenario, hot gases (red arrows) are shielded from the joint by the zinc-chromate putty. On the right, immense pressure creates a blowhole in the putty, allowing the O-rings to move into the positions needed to seal the joint as the gap between tang and clevis expands. Through the blowhole, gases penetrate and wear away the O-rings.

Boisjoly had noticed that O-rings eroded, to an extent, in this fashion previously. NASA and Thiokol, however, decided that, since the O-rings were not completely eroded, there was minimal risk. Boisjoly's concern was that the low launch temperature would cause the O-rings to contract and further compromise their sealing value.

This diagram is a cross section. In actuality, the joint itself, tang, clevis, and O-rings have the circular shape of the SRB.

Sources:Report to the President by the Presidential Commission on the Space Shuttle Challenger Accident.

Photographs adapted from pages 112, 52, and 57 of the Presidential Commission Report.

Cite this page: "What Went Wrong" Online Ethics Center for Engineering 8/29/2006 OEC Accessed: Tuesday, September 19, 2017 <>